The invention relates to a gas turbine engine control method and apparatus. More specifically, the invention relates to an improved system for controlling the fuel/air mixture supplied through a combustion chamber of a gas turbine.
In large capacity gas turbine engine systems, such as used for generation of electric power, a gas turbine engine is designed to operate at a relatively constant rotational speed of the turbine which drives an electric generator unit. The load on the turbine engine varies throughout the day due to changes in electrical power demands. For maximum efficiency and minimum emission of NO.sub.x compounds, it is desired to maintain optimum fuel to air ratio mixtures supplied to the combustion chamber over all operating ranges.
U.S. Pat. No. 4,529,887 to Johnson discloses a prior art arrangement of a gas turbine system of the type which the present invention is directed toward improving.
In conventional turbine systems of the type referred to above, the fuel flow rate is controlled by varying the opening of compressor inlet guide vanes in response to load changes on the turbine. During decreases in load on the turbine, the fuel flow rate is decreased and the opening of compressor inlet guide vanes is controlled to be closed to reduce the supply of air. However, problems arise with such systems, especially during decreasing loads, since the closing of the compressor inlet guide vanes lags behind the decrease in the fuel flow supply, leading to an excessive air supply in the fuel-to-air ratio of the mixture supplied to the combustion chamber. The resultant imbalance in the fuel/air ratio supplied to the combustion chamber leads to problems of combustion instability, which affects the emissions and the efficiency of the turbine engine systems.
Also, with prior art arrangements, during low atmospheric temperature conditions in the winter season, for example, the effective air flow rate increases due to its high density. Thus, especially under low atmospheric temperature conditions, even more excessive air is mixed with the fuel during load decreases due to the delayed closing of the compressor inlet guide vanes during such load decreases.
An object of the present invention is to provide a method and apparatus for controlling a gas turbine engine system with high reliability while improving the combustion stability during decreasing load. A further object is to optimize combustion stability throughout changing exterior temperature conditions during, for example, operation in a winter season as compared to a summer season.
These objects are achieved according to the present invention by providing a control method and system which includes different operating control methods during decreasing loads below a predetermined rated load operation range of the gas turbine system. In especially preferred embodiments, during the rated load operation, the inlet guide vanes are in the fully open condition and the combustion fuel air mixture is controlled by regulating only the fuel flow rate as a function of detected turbine exhaust gas temperature changes. During the rated load operation range of the system, adjustments in the fuel supply are made in response to large turbine exhaust temperature deviations from a present desired turbine exhaust gas temperature of, for example, 5.degree. C., with the adjusting of fuel flow made at a relatively fast changing rate corresponding in preferred embodiments to turbine exhaust temperature correcting rate of 1.degree. C./s (1.degree. C./second). This is a so-called "exhaust gas temperature control" mode of operation, hereafter EGT control. During decreasing load operations, when turbine exhaust temperature falls a pre-set amount below a pre-set temperature below a predetermined desired temperature, the control is switched from-the EGT control to "compressor inlet guide vane control" (hereafter IGV control) with guide vane opening changes being initiated and implemented at a different changing rate and in response to a lower temperature deviation from the desired turbine exhaust gas temperature than is the case with EGT control. The system of the present invention advantageously assures the proper fuel/air ratio, during load fluctuations, and especially during load reductions, because the IGV control effects a more precise and more timely reduction in combustion air supply than with prior arrangements.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.